The present invention relates to communication systems.
Some communications systems, for example, digital subscriber line (“DSL”) systems, transmit signals in a differential mode using two conductors that can be formed in a twisted pair transmission line. Twisted pair transmission lines can be co-located with other transmission lines in a binder (e.g., a cable). Each twisted pair forms a loop (i.e., when connected to a termination device) and can suffer from crosstalk interference (or simply crosstalk) when bundled with other pairs.
On loop lengths short enough to support Asymetric Digital Subscriber Line 2 plus (“ADSL2+”) or Very-high bit rate Digital Subscriber Line (“VDSL”) data rates greater than 12 Mbps, far-end crosstalk interference (“FEXT”) can become the dominant contributor to degrading the signal to noise ratio at a victim receiver. In conventional systems, FEXT can limit the rate and reach of DSL signals greater than 8 Mbps.
Recent efforts to improve DSL performance through crosstalk reduction have focused on so called multiple-input, multiple output (“MIMO”) processing schemes. These schemes attempt to characterize the crosstalk relationship between twisted pairs. The primary drawbacks of a conventional MIMO scheme are computational complexity, error due to ingress noise and signal attenuation, requirements that both ends of the communication network be upgraded, and long lag time associated with updating the MIMO information.
Efforts to improve network performance also include work in the area of dynamic spectrum management (“DSM”), in which the frequency and power usage of transmission lines are adjusted to achieve a desired performance in a manner that is least detrimental to other transmission lines. DSM is generally a scheme to coordinate different DSL lines within a binder in a way that avoids crosstalk. In the highest performance mode, the transmitters' power spectrum densities are adjusted dynamically to optimize overall network performance. Conventional DSM is an avoidance scheme and not a cancellation scheme and, thus, has limited effectiveness as the density of DSL signals in a binder increases.
Improved cabling is another area of focus for improving network performance. Although improved cabling can provide better immunity from disturber transmission lines and reduce crosstalk interference to other transmission lines and services, the use of improved cabling may be infeasible in many situations. Replacing existing cables can be expensive (due to material and labor costs), time consuming, and disruptive to service. Furthermore, existing conduits in buildings are often not large enough to accommodate the larger cables that have improved shielding, and replacing these conduits may be difficult or impossible.